It is common during medical situations to monitor a patient's heart by sensing the electrical excitation pulses, i.e., the electrocardiogram (ECG) signals, that cause the contraction of the heart's ventricles and atria. ECG signals are commonly detected by an ECG signal detection device that includes a pair of electrodes each incorporated into an adhesive pad designed to be placed on a patient's chest. Leads connect the electrodes to the inputs of a differential input amplifier. The differential amplifier detects the slight ECG signals associated with the contraction of the heart and amplifies the ECG signals so that the ECG signals can be analyzed and/or displayed for analysis by a medical instrument such as a defibrillator. Because the magnitude of the ECG signals are relatively low and because relatively high patient transthoracic impedance makes isolating the ECG signals difficult, the amplification necessary to display ECG signals is relatively high. Relatively high amplification makes the electrode leads of the ECG signal detection device susceptible to outside electrical noise created by sources such as overhead lights or patient capacitance to earth. The outside electrical noise created by nearby power sources (50–60 Hz) can be nullified by a band-pass filter that passes ECG signals over a lower bandwidth (2–40 Hz). Employing a low frequency band-pass filter has the disadvantage of requiring operators to “stand clear” of the patient so as to not affect the reading of the patient being analyzed. Recently, medical instruments have begun analyzing ECG signals using various Shock Advisory System (SAS) algorithms. Such algorithms are incompatible with the low pass filtering of ECG signals because they require signal frequencies above the upper cut-off frequency of the low pass filter passband in order to properly diagnose ECG signals and, thus, a patient's condition. However, as noted above, raising the cut-off frequency of the passband results in greater susceptibility to outside electrical noise. Outside electrical noise can create false ECG signals that could be misinterpreted by an operator or SAS algorithm. Incorrect interpretation of the ECG signals can result in inappropriate treatment.
The effect of outside electrical noise associated with ECG signals can be reduced if not entirely eliminated by applying a third electrode to a patient's chest and connecting the third electrode to a driven reference input of the differential input amplifier via a suitable driven reference lead circuit. The capability of a differential input amplifier to eliminate the effect of outside electrical noise is expressed in decibels as a common mode rejection ratio (CMRR). As a differential amplifier's CMRR becomes larger, it is less susceptible to outside electrical noise, i.e., more outside electrical noise detected by the electrode leads is rejected.
As stated above, two of the electrode leads of a typical three-electrode lead differential amplifier input system form signal detection electrode leads. The third electrode lead functions as a driven reference electrode lead. The third electrode lead provides a low-resistance path for grounding outside electrical noise signals, significantly reducing the effect of outside electrical noise on ECG signals. When a third electrode lead is used as a driven reference, the CMRR of a typical differential input amplifier is increased by approximately 30 dB.
As a result, many ECG signal devices include a third electrode and a third electrode lead for use as a driven reference for the differential amplifier receiving the ECG signals. In the past, the three-electrode leads have been separate and were attached to the patient's chest using three separate adhesive pads each housing an electrode. That is, the patient end of each electrode lead is attached to an electrode mounted in a separate adhesive pad. Most such adhesive pads are disposable, i.e., only intended for use with one patient.
The application of three separate electrode pads to a patient is disadvantageous in emergency situations. It is inconvenient to fix a third electrode pad to a patient when time is critical. Additionally, when a defibrillator unit is being used for both ventricular defibrillation and ECG signal monitoring, fixing a third electrode pad to a patient while holding two defibrillator pads or paddles, each of which also functions as an ECG signal detection electrode for an ECG signal device, is difficult at best. Even though a third or reference electrode lead reduces the possibility of ECG signal error, it has been found that the practice of applying a separate reference electrode lead to a patient is too inconvenient to be acceptable during emergency situations.
Thus, there exists a need for methods and apparatus that overcome these disadvantages. The present invention is directed to providing such methods and apparatus.